Chapter 2

Mechanisms of Evolutionary Change

Selection in Darwin’s Theory of Evolution

• Artificial selection• Natural selection

– No ultimate goal– Current environmental parameters– Traits that increase likelihood of survival

• Sexual selection– Traits that increase likelihood of mating– May actually decrease individual’s survival

Differential Reproductive Success

• Not just how many offspring you produce

• Number of offspring that survive to reproduce themselves

Mendelian Genetics

• Phenotype, genotype

• Gene, allele, locus

• Homozygous, heterozygous

• Dominant, recessive

• Particulate, not blended, inheritance

• Born with the genes you pass on

Mendelian Genetics

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Beyond Mendel

• Not everything in genetics follows Mendel’s basic principles

• Linked genes• Chromosomal crossing over during meiosis

– Recombination, or “chromosomal mutation”

• Autosomal and sex chromosomes• Gene mutation

– Neutral, detrimental, beneficial

– Only one source of genetic variability

Accounting for Variability

• Sexual reproduction (“Mendelian variation”)

• Recombination (aka “chromosome mutation”)

• Gene mutation

Polygenetic Effects

• Not all genes fully dominant/recessive

• Pleiotropy: genes with multiple phenotypic effects

• Turning a gene “on” or “off”– “Modifier” genes– Gene-gene interaction– Environment

Genomic Imprinting

• Usually, parental contributor of an allele is irrelevant for gene expression

• With imprinted genes, parental contributor matters– <1% of genes

– About 80 discovered so far in humans

– Mostly involved in embryonic and placental development

• Parental Conflict Hypothesis– Moore & Haig (1991)

Igf2 Gene• Paternally expressed

– Gene only activates if it is contributed by the father (i.e., from the sperm)

• Influences embryo growth– Insulin-like growth factor --> bigger embryo

• Male– “Wants” largest embryo possible– Cost to mother doesn’t affect father’s future

reproductive output

• Female– “Wants” large embryo, but not too large– False-receptor imprinted gene; blocks additional

growth hormone

DNA

• Deoxyribonucleic acid• Double helix• Nucleotide

– Backbone– Base pairs: adenine-thymine, cytosine-guanine

• Amino acids coded for by triplets of base pairs (a codon)

• The genome

Book Metaphore of Genome

• There are 23 chapters, called CHROMOSOMES• Each chapter contains several thousand stories,

called GENES• Each story is made up of paragraphs, called

EXONS, which are interrupted by advertisements, called INTRONS

• Each paragraph is made up of words, called CODONS

• Each word is written in letters called BASES

What Genes Do

• Amino acids, when strung together, code for polypeptide production

• Proteins formed from multiple polypeptides linked together (“transcription”)

• How this translates to physical and/or behavioural traits is highly interactive, depending on environment– E.g., sugars in cell can affect polypeptide

folding, altering proteins that can be made

30,000

• Human Genome Project (2001)

• Mapped loci on all chromosomes

• Don’t actually know what all 30,000 genes do

• Don’t forget polygenetic and other environmental effects

Heritability

• Degree to which phenotypic variance in population is due to genotypic variance

• Genetic rather than environmental influences

• Greater a trait’s heritability, the greater the genetic control over it (i.e., the less environmental influence)

Source of Control

• Even high heritability traits can still be affected by environmental factors

• Compare phenotypic variation in people of differing degrees of relationship (e.g., monozygotic, dizygotic twins)

• Correlations– Zero to one, positive or negative

• Even high heritability traits only show correlations in the 0.5-0.7 range

Variation in the Population• Consider from perspective of an

individual’s offspring• Proximate level interpretation

– Sexual reproduction, chromosomal and gene mutations

• Ultimate level interpretation– Change in environment less likely to eliminate

all offspring– Asexual vs. sexual reproduction– Disease and parasitism

Evolution is Generational• Shifts in gene frequencies based on generations

– ~300,000 generations b/t common ancestors of modern chimpanzees and humans, (~5 million years)

– Same number of bacteria generations in ~25 years

• Environment can change rapidly– Time scale issues– Short-term shifts, long-term stability

• Under certain conditions, gene frequencies can change “moderately” rapidly

• Variability in population as well as learning ability allows for adaptation to changes

Non Natural Selection Evolution

• Not all evolution due to natural (or sexual) selection

• Gene flow– Animal moves from one population to another– If its genotype confers an advantage, this can

alter the gene frequencies in the local population “rapidly”

• Genetic drift– Changes in population due to chance, because

traits not selected for/against (i.e., neutral)– Generally only significant in small populations;

each mating act has larger influence on gene frequencies of population

• Founder effect– Subset of genetic drift– New population is established by one or a few

individuals; biases initial gene frequencies

Other Factors

• Disease• Climate change• Volcanism• Asteroid impacts• Human intervention on environment, species, etc.• Generally, sudden large scale changes that operate

too rapidly for natural and sexual selection to adapt organism (i.e., change gene frequencies)

Levels of Selection: Group or Individual?

• Group

• Species, population, community, etc.

• Initially seemed to explain altruism– Altruism: sacrificing your own fitness to

improve the fitness of others

• “Greater goodism”

Group Selection

• Evolution of traits benefiting population, not individual

• Sacrifice/loss for individual• Williams (1966)

– Group selection can seldom counter force of individual selection

• Don’t necessarily need to invoke group selection

Individual as the Level of Selection

• Individual generally seen as the level on which evolution operates

• Individuals do/don’t pass their genes on– Gene frequencies in population shift over time

• Direct (“classical” or “individual”) fitness– Genes passed on specifically by individual

reproducing

Inclusive Fitness

• William D. Hamilton (1964)• Share genes with close relatives (kin)• Can increase fitness by assisting relatives’

reproductive success• Sum of

– Individual’s reproductive success (direct fitness)– Individual’s success in promoting the successful

reproduction of relatives

• Can explain many cases of altruism

r, Coefficient of Relatedness

• Probability that two individuals share the same gene because they inherited it from a common ancestor

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Theory of Kin Selection

• Altruism: sacrifice for the benefit of another• “Hamilton’s Rule”• rB > C

– Where B = benefit to recipient– C = cost to yourself– r = coefficient of relatedness

• EP explains altruism via kin selection (inclusive fitness)

Reciprocal Altruism

• Altruistic acts between non-relatives• A low cost to the donor and a high benefit to the

recipient• In future, the debt is repaid (i.e., “reciprocated”)• Both donor and benefit benefit• Requires stable social group, ability to identify

individuals, longer lifespans• Not basic altruism

Error on p. 53 Text• “Imagine, for example, that we are living on the plains of the

Serengeti in Africa and that I have just killed a wildebeest. There is more meat than I could possibly eat before it either goes off in the heat… or is stolen by a clan of hungry hyenas. You may be starving but I can save your life at very little cost by giving you meat that is left over when I’ve had my fill.”

• Bad wording in the example• Actually “tolerated theft”• No real cost to donor here: “…more meat than I could possibly

eat…”• Must be some cost to donor for reciprocal altruism• Example would work if donor gave some meat that he could still

have eaten

Competitive Altruism

• Sacrifice, or appearance of sacrifice, to gain future personal benefit

• Attention, possible mating opportunities, resources, status

• “Donor” gains more than he/she sacrifices• Not basic altruism

Selfish Gene

• Richard Dawkins• Gene’s-eye view

– Metaphor

• Replicators and vehicles• Genes are eternal, individuals are ephemeral• Can gene be actual level of selection?

– 30,000 genes in a human– Individual genes canceling out each others’ attempts to

behave selfishly?– Behaviour occurs at the level of the individual, not the

gene

Multi-Level Selection

• Elliot Sober and David Sloan Wilson

• Group, individual, and gene selection

• Group cooperation, for example, increases reproduction of individuals in group– But, often group members are genetic relatives,

so…

• Levels of causation seem to be an issue here

• Recent, and actively debated theory